Negative Entropy Of Activation Research Articles

Reaction Kinetics of the Benzylation of Adenine in DMSO: Regio-Selectivity Guided by Entropy.

The factors governing the regio-selectivity of the alkylation of adenine have been of interest for many years due to the biological importance of adenine derivatives, however, no reaction kinetic studies have been conducted. Herein, we report the rate constants and activation parameters of the benzylation of adenine under basic conditions in DMSO in the absence and presence of 15-crown-5 ether using real-time 1H NMR spectroscopy. The reaction is second-order for the formation of the N9- and N3-benzyladenine products, with a regio-selectivity factor 2.3 in favour of the N9-adduct. The Gibbs free energy of activation amounts to 87±2 kJ mol-1 for both reactions. The formation of the N9-adduct is more activated by 7 kJ mol-1, but its effect is offset by a less negative activation entropy, demonstrating that the long-contested reason for the regioselectivity in the benzylation of adenine is dominated by compensation of entropy and enthalpy in the transition state. The kinetic parameters obtained in the presence of the 15-crown-5 ether indicate that the crown ether forms a complex with an adenine-sodium ion-pair, increasing the activation barrier. However, the Gibbs free energy in the absence and presence of the crown ether remains constant.

Application of ANN and RSM for Rhodamine B and Safranine-O Decolorization on Zinc-Carbon Battery Waste Derived Ag/CoFe-LDH/rGO Catalyst.

The present work is first aimed at recovering graphite from carbon rods of waste zinc-carbon (Zn-C) batteries for applications such as wastewater treatment, in order to contribute to the development of a sustainable environment. Then, a composite material, cobalt-iron layered double hydroxide combination with reduced graphene oxide, and with subsequent Ag nanoparticles deposition via NaBH4 reduction method (Ag/CoFe-LDH/rGO) was prepared for the catalytic activity of Rhodamine B (RhB) and Safranine-O (SO) as model contaminants from aquatic media. The catalytic activity of RhB and SO by Ag/CoFe-LDH/rGO in the presence of NaBH4 was studied to model and optimize the process parameters (NaBH4 amount, reaction time, initial dye concentration (Co), and catalyst dosage) via central composite design (CCD)-response surface methodology (RSM). Also, an artificial neural network (ANN) model was developed to estimate the catalytic activity of each dye using an RSM data set. The catalytic activities of 99.54% and 99.96% were obtained for RhB and SO dyes, respectively, under the optimal conditions: NaBH4 amount 12.32 mM, reaction time 3.19 min, Co 33.46 mg/L, and catalyst dosage 1.24 mg/mL for RhB dye; NaBH4 amount 16.76 mM, reaction time 3.06 min, Co 15.10 mg/L, and catalyst dosage 1.46 mg/mL for SO dye. The optimum conditions of process parameters by ANN with gray wolf optimizer (GWO) were in good agreement with the points determined the RSM-CCD. These results demonstrate that RSM and ANN approaches can be applied practically and efficiently to maximize the catalytic activity of RhB and SO by Ag/CoFe-LDH/rGO in the existence of NaBH4. On the other hand, from the kinetic and thermodynamic studies, the positive activation enthalpy, ΔH# and the negative activation entropy, ΔS# values for each dye demonstrated that the catalytic performance was endothermic and less random at the solid/liquid interface.

Effect of agitation speed on methylene blue discoloration kinetics via ambient air cold plasma

AbstractThis research investigates the kinetics of methylene blue (MB) discoloration using ambient air cold plasma, with a focus on the impact of agitation speed (100 and 750 rpm). The study revealed pseudo‐first‐order kinetics for MB discoloration, pinpointing optimal conditions at 35.00°C and 100 rpm. These parameters minimized half‐life times, correlated with observed kobs values. A decreasing pH trend, more pronounced at 750 rpm, was attributed to increased acidic nitrogen species (HNO3 and HNO2) production, adversely affecting dye discoloration. Concurrently, enhanced electrolyte concentration was noted from rising conductivity due to plasma production of reactive species followed by solubilization in the aqueous phase. The calculated thermodynamic activation parameters comprised: Ea = 7.96 kJ mol−1, ΔH‡ = +5.53 kJ mol−1, ΔS‡ = −253.23 J K−1 mol−1, and ΔG‡ = +79.77 kJ mol−1 (100 rpm); and Ea = 12.94 kJ mol−1, ΔH‡ = +13.78 kJ mol−1, ΔS‡ = −239.06 J K−1 mol−1, and ΔG‡ = +80.58 kJ mol−1, (750 rpm). The lowest Ea and ΔG‡ values at 100 rpm reinforced lower agitation favoring the reaction. The study demonstrated a linear decay of the reaction rate constant with the square root of ionic strength. This result, besides the negative activation entropy and moderate activation enthalpy led to a proposition to the determinant step for the transition state formation, involving an associative step between a solvated electron and the protonated substrate. The optimal dye discoloration rate and energy yield were observed at 35.00°C and 100 rpm, with values of 97.2% and 3.371 × 10−2 g kW−1 h−1, respectively.

Eco-friendly corrosion inhibition of steel in acid pickling using Prunus domestica Seeds and Okra stems extracts

The corrosion inhibition process during acid pickling in petroleum industries often employs toxic chemicals, facing stringent environmental regulations. This study explores a sustainable alternative by investigating the inhibitory effects of eco-friendly food waste extracts, namely Prunus domestica seeds (P.S) and Okra stems (O.St), on B7 grade steel corrosion in 1.0 M HCl aqueous solution, simulating an acid pickling environment. The corrosion inhibition efficiency was estimated via both electrochemical methods, including electrochemical impedance spectroscopy and potentiodynamic polarization techniques, and chemical methods, such as weight loss measurements. Fourier transform infrared spectroscopy (FTIR) and Gas chromatography-mass spectrometry (GC-MS) elucidated the extracts' main functional groups and potential chemical constituents. Remarkably, both extracts exhibited a maximum inhibitory efficiency of up to 80 %. Adsorption isotherms were employed to analyze the inhibition mechanisms, including Langmuir and Flory–Huggins models, alongside the Kinetic-thermodynamic model. Activation parameters have been obtained using Arrhenius and transition state equations. The synthesis of the activated complex is characterized as an endothermic process, as evidenced by the positive values of enthalpy of activation (ΔH*). Conversely, negative entropy of activation (ΔS*) values indicate that the activated complex signifies a process of association rather than separation. Quantum chemical calculations identified potential active inhibitor compounds within the extracts. The quantum parameters, including ionization potential, electronegativity, softness, and hardness of the chemical constituents of the extracts, were calculated using DFT with the B3LYP/6–31 G(+) method. This study underscores the innovative approach of utilizing waste food extracts as corrosion inhibitors, addressing environmental concerns while offering practical implications for industrial applications.

The effect of substituent position and solvent on thermal Z‒E isomerization of dihydroquinolylazotetrazole dyes: kinetic, thermodynamic, and spectral approaches.

Kinetic and thermodynamic parameters have been investigated for the thermal Z‒E isomerization of dihydroquinolylazotetrazole dyes with alkyl substituents (Me, t-Bu, and Adm) at positions 1 (dyes 2) and 2 (dyes 3) of the tetrazole moiety in two solvents of different polarity, acetonitrile (MeCN) and toluene. The experimental results show crucial dependence of these parameters on a substituent position in the tetrazole moiety and on a solvent. For dyes 2, Eact and ΔH‡ are lower in MeCN than in toluene that results in a high increase in the lifetimes of the Z isomers: from milliseconds in MeCN to minutes in toluene. For dyes 3, the difference in Eact and ΔH‡ in the two solvents is opposite: Eact and ΔH‡ are by more than 20kJmol-1 higher in MeCN, nevertheless, the rate constants for 3 in toluene are comparable with those in MeCN at the ambient temperature and the difference in the behavior is determined by the value of negative entropy of activation. Quantum-chemical calculations of the thermal Z‒E isomerization show the possibility of the process to occur via crossing from the S0 to the thermally induced T1 state. The contribution of this path is highest for 3 in toluene. The analysis of the absorption spectra demonstrates that for the E isomers, the n‒π* and π‒π* transitions are within the long-wavelength absorption band and their positions relative each other are opposite in the solvents: the n‒π* transition is blue-shifted relative to the π‒π* transition in MeCN and is red-shifted in toluene.

The influence of electronic effects on the substitution reactions of N,N′-donor cis-palladium(II) complexes with bidentate (pyridin-2-yl)methyl-aniline ligands

The substitution behaviour of bidentate N,N′-donor (pyridin-2-yl)methyl-aniline chelates with different substituents on the mononuclear Pd(II) complexes was investigated. The complexes were synthesized and characterized using 1H NMR, 13C NMR, FTIR, LC–MS, and elemental analysis. The study explored the kinetics and mechanistic behaviour of N,N′-pyridyl Pd(II) complexes, viz. dichloro-(N-((pyridin-2-yl)methyl)aniline)palladium(II) (PdL1), dichloro-(4-fluoro-N-((pyridin-2-yl)methyl)aniline)-palladium(II) (PdL2), dichloro-(4-methoxy-N-((pyridin-2-yl)methyl)aniline)-palladium(II) (PdL3) and dichloro-(4-ethyl-N-((pyridin-2-yl)methyl)aniline)-palladium(II) (PdL4). The effect of concentration and temperature on substitution behaviour of coordinated chloride ligands by three bio-relevant thiourea nucleophiles, viz. thiourea (TU), N,N′-dimethylthiourea (DMTU) and N,N,N′,N′-tetramethylthiourea (TMTU), of different steric demands was studied in a 0.1 M (LiCl) solution of ultra-pure water under pseudo-first order conditions using standard Stopped-Flow and UV–Visible spectrophotometric techniques. The substitution of the chloride ligands from the Pd(II) metal by thiourea nucleophiles was a two-step reaction, where the chloride trans to the pyridine ligand was substituted first due to the strong trans effect on the pyridine ring compared to the amine group. The rate of substitution of the chloride by thiourea nucleophiles increased with the presence of an electron-withdrawing substituent and decreased when an electron-donating group was attached to the para position of the phenyl moiety. The negative activation entropies and positive activation enthalpy for all the substitution reactions support an associative mode of substitution mechanism.

Detailed Understanding of Solvent Effects for the Cationic Ring-Opening Polymerization of 2-Ethyl-2-oxazoline

Polymerization of 2-ethyl-2-oxazoline (EtOx) has often been in the spotlight for fundamental studies of poly(2-alkyl/aryl-2-oxazoline)s (PAOx) polymerization, especially initiator screening, solvent screening, and copolymerization trends. In this work, we build on previous observations of solvent effects on the cationic ring-opening polymerization (CROP) of EtOx, with additional experimental observations of previously unreported solvents to expand the explored parameter space. Our objective is to find solvents with the lowest activation energy (Ea) and higher Arrhenius preexponential factor (A), which will allow us to produce narrow molar mass distributions at higher molecular weights, in the least time. To achieve this, we examined the various single factors like Dimroth ET(30) values, the Kamlet–Abraham–Taft (KAT) linear free-energy relationship (LFER) equation(s), and the Catalan LFER equations. Only one of Catalan’s equations sufficiently disentangled dipolarity and polarizability to give a good fit due to contradictory effects. It was found that solvent nucleophilicity, electrophilicity, and polarizability affected the Ea, but not dipolarity. All four factors affected the A. This indicates that the Ea is minimized in solvents that do not solvate ions well (i.e. force ion-pairing), and A was minimized in more dipolar solvents that solvate the polymer chains well. A strongly negative activation entropy (ΔS‡) shows that the propagation reaction is associative. The Catalan LFER allows for the prediction of Ea, A, ΔH‡, and ΔS‡, and the derived kp, across a broad range of solvents.

Structural Study of Model Rhodium(I) Carbonylation Catalysts Activated by Indole-2-/Indoline-2-Carboxylate Bidentate Ligands and Kinetics of Iodomethane Oxidative Addition

The rigid-backbone bidentate ligands Indoline-2-carboxylic acid (IndoliH) and Indole-2-carboxylic acid (IndolH) were evaluated for rhodium(I). IndoliH formed [Rh(Indoli)(CO)(PPh3)] (A2), while IndolH yielded the novel dinuclear [Rh1(Indol’)(CO)(PPh3)Rh2(CO)(PPh3)2] (B2) complex (Indol’ = Indol2−), which were characterized by SCXRD. In B2, the Rh1(I) fragment [Rh1(Indol’)(CO)(PPh3)] (bidentate N,O-Indol) exhibits a square-planar geometry, while Rh2(I) shows a ‘Vaska’-type trans-[O-Rh2(PPh3)2(CO)] configuration (bridging the carboxylate ‘oxo’ O atom of Indol2−). The oxidative addition of MeI to A2 and B2 via time-resolved FT-IR, NMR, and UV/Vis analyses indicated only Rh(III)-alkyl species (A3/B3) as products (no migratory insertion). Variable temperature kinetics confirmed an associative mechanism for A2 via an equilibrium-based pathway (ΔH≠ = (21 ± 1) kJ mol−1; ΔS≠ = (−209 ± 4) J K−1mol−1), with a smaller contribution from a reverse reductive elimination/solvent pathway. The dinuclear complex B2 showed the oxidative addition of MeI only at Rh1(I), which formed a Rh(III)-alkyl, but cleaved the bridged Rh2(I) site, yielding trans-[RhI(PPh3)2(I)(CO)] (5B) as a secondary product. A significantly smaller negative activation entropy [ΔH≠ = (73.0 ± 1.2) kJ mol−1; ΔS≠ = (−21 ± 4) J K−1mol−1] via a more complex/potential interchange mechanism (the contribution of ΔS≠ to the Gibbs free energy of activation, ΔG≠, only ±10%) was inferred, contrary to the entropy-driven oxidative addition of MeI to A2 (the contribution of ΔS≠ to ΔG≠ ± 75%).

Solving the Azobenzene Entropy Puzzle: Direct Evidence for Multi-State Reactivity.

A solution to the azobenzene "entropy puzzle" [ J. Phys.: Condens. Matter, 2017, 29, 314002] is provided. Previous computational studies of the thermal Z → E (back-)isomerization of azobenzene could not describe the experimentally observed large negative activation entropies. Here it is shown that the experimental results are only compatible with a more complicated multistate rotation mechanism that involves a triplet excited state. Using nonadiabatic transition state theory, close to perfect agreement is achieved between all calculated and experimental Eyring parameters. We also provide new experiments that indicate the presence of a noticeable external heavy-atom effect, which is a direct result of spin-orbit coupling effects being important in the proposed mechanism. These results suggest a reexamination of the mechanisms of related thermal double bond isomerizations in other systems in cases when an excited state of triplet (or other) multiplicity becomes thermally accessible during a rotation process.

Kinetics of Ring Closure Thiazole Cyclization Using 3-Chloroacetylacetone with Thioamides

Background: Thiazoles are an important heterocycle which exhibits various bioactivities. Thiazole ring is found in core structure of various medicinally useful compounds. It shows biological activities like anti-inflammatory, antibacterial, antifungal, antioxidant, cytotoxicity, anticonvulsant, insecticidal, anticancer and antitumor. Thioamides are reported to show bioactivities like antibacterial, antileukemia, antifungal, anti-arthritic, antimalarial and cytotoxic. Hantzsch synthesis is one of the most efficient reactions for thiazole formation which is a cyclization reaction between thioamides and α-halo carbonyl compounds. A very few reports on kinetic investigation of thiazole formation are available. Previously we have studied thiazole formation using 3-chloroacetylacetone and substituted thioureas by pH metric study. The reaction between 3-chloroacetylacetone and Thioamides is not reported yet. The present work deals with kinetics and mechanistic study of ring closure of thiazole using thioamides and 3-chloroacetylacetone. The reaction has been carried out in water-ethanol system at 303K. The kinetic investigations are carried out pH metrically. Nature of reaction is explained on the basis of thermodynamic parameters. Reaction products are isolated and characterized on the basis of spectral data. The effect of temperature on reaction rate is studied at 30°C to 50°C. Thermodynamic parameters are evaluated. Objective/s: To determine the order of reaction. • To determine the rate of reaction. • To evaluate the rate of reaction at various temperatures. • To determine the rate of reaction in different media. • To study the effect of dielectric constant, ionic strength etc. • To observe calculation of energy of activation, enthalpy of activation, entropy of activation and free energy of activation. • To propose reaction mechanism considering various thermodynamic parameters. Method: Measuring the change in pH, the concentration of H+ ion will be determined and accordingly rate of reaction will be calculated. Result: Van’t Hoff’s differential method and the stoichiometric study suggested that one mole of Thioamides reacts with one mole of 3-chloroacetylacetone. The rates of reaction are measured at different concentration of Thioamides at constant concentration of 3-chloroacetylacetone. The plot of log (dc / dt) against log [3-chloroacetylacetone] and log [ Thioamides] is also straight line and slope of the plot is nearly one with respect to both. At five different temperatures second order rate constants are determined. The energy of activation (Ea) is determined by plotting graph of logk versus 1/T and other thermodynamic parameters are calculated. The negative entropy of activation (∆S*) of this reaction specifies rigid nature of the transition state. The entropy of activation (∆S*) is negative value and it specifies that less stable noncyclic reactants convert into stable cyclic product. Free energy of activation (∆G*) is calculated [Table 1], Graph 1. Reacting species are oppositely charged according to salt effect [Graph 2]. In water-isopropyl alcohol system reaction is fast than water-ethanol system [Graph 3]. Conclusion: • Using Van’t Hoff’s differential method the rate of reaction between 3-chloroacetylacetone and Thioamides is found to be one with respect to both components. • The proposed rate law also shows that the order of reaction is two. • Negative entropy (∆S*) shows that from open chain compound cyclic compound is formed. Nucleophilic addition and addition–elimination reaction take place. • Oppositely charge species are present in reaction. • Rate increases with increase in dielectric constant. Isopropyl alcohol-water system shows high-rate constant as compared to Ethanol-water system.

Malonic Anhydrides, Challenges from a Simple Structure.

After many years of unsuccessful attempts, monomeric malonic anhydrides were prepared by ozonolysis of ketene dimers, a procedure validated by model studies. The structure proof relied most heavily on IR absorption at 1820 cm-1 and a Raman band at 1947 cm-1. Malonic anhydrides are unstable, decomposing below room temperature to a ketene plus carbon dioxide. Surprisingly, according to kinetic studies, the dimethyl derivative is slightly less unstable than the parent, and the monomethyl is the fastest to decompose, with an enthalpy of activation of only 12.6 kcal/mol. Computations rationalize this behavior in terms of a concerted [2s + 2a] cycloreversion that requires a more highly organized transition state, as also manifested by a negative entropy of activation.

The Activation Parameters of a Cold-Adapted Short Chain Dehydrogenase Are Insensitive to Enzyme Oligomerization.

The structural principles of enzyme cold adaptation are of fundamental interest both for understanding protein evolution and for biotechnological applications. It has become clear in recent years that structural flexibility plays a major role in tuning enzyme activity at low temperatures, which is reflected by characteristic changes in the thermodynamic activation parameters for psychrophilic enzymes, compared to those of mesophilic and thermophilic ones. Hence, increased flexibility of the enzyme surface has been shown to lead to a lower enthalpy and a more negative entropy of activation, which leads to higher activity in the cold. This immediately raises the question of how enzyme oligomerization affects the temperature dependence of catalysis. Here, we address this issue by computer simulations of the catalytic reaction of a cold-adapted bacterial short chain dehydrogenase in different oligomeric states. Reaction free energy profiles are calculated at different temperatures for the tetrameric, dimeric, and monomeric states of the enzyme, and activation parameters are obtained from the corresponding computational Arrhenius plots. The results show that the activation free energy, enthalpy, and entropy are remarkably insensitive to the oligomeric state, leading to the conclusion that assembly of the subunit interfaces does not compromise cold adaptation, even though the mobilities of interfacial residues are indeed affected.

Mechanism of halide exchange in reactions of CpRu(PPh3)2Cl with haloalkanes

Kinetic measurements of the reaction between CpRu(PPh3)2Cl (1a) and 1-bromobutane reveal a nearly first order dependence on the concentration of haloalkane and a negative entropy of activation, ΔS† < 0.

Kinetic and Computational Analysis of CO Substitution in a Dinuclear Osmium Carbonyl Complex: Intersection between Dissociative and Dissociative-Interchange Mechanisms.

The mechanism for the CO substitution reaction involving the diosmium carbonyl sawhorse complex Os2(μ-O2CH)2(CO)6, which contains an Os-Os single bond, two axial CO ligands, and four equatorial CO ligands, was investigated experimentally and theoretically. Kinetic measurements show 13CO axial substitution proceeding by a dissociative reaction that is first-order in the complex and zero-order in 13CO but with an unexpectedly negative entropy of activation. The corresponding electronic structure calculations yield an enthalpy of activation for axial CO dissociation that is much larger than that determined by the kinetic experiments, but in agreement with the complex's stability with respect to CO loss. Additional calculations yield a dissociative interchange transition state whose free energy, enthalpy, and entropy of activation are in good agreement with those obtained from the kinetic measurements for the apparently dissociative substitution. These results point to an exchange reaction mechanism that is surprisingly close to the poorly understood transition from a dissociative mechanism with a CO-loss intermediate to a dissociative interchange mechanism with a transition state involving both the entering and the leaving COs. The key to explain these findings is provided by the vibrational analysis, which shows very low energy wagging motions for the axial COs. Thus, the incoming CO only displaces the outgoing CO when the complex has an outgoing CO near the wag's turning point. This dissociative interchange mechanism predicted by the calculation explains the unexpected combination of kinetics and stability characteristics. Kinetics reveals that the reaction is first-order in the Os dimer with a negative Eyring entropy, while a stability study shows that the Os dimer's decomposition rate is several orders of magnitude slower than CO exchange.

SULFONYL CHLORIDES - NOVEL SOURCE OF FREE RADICALS

Novel free radicals source based on sulfonyl chlorides is discovered. The radical mechanism is confirmed by 2,3-dimethyl-2,3-diphenylbutane formation under chlorosulfonated polyethylene heating in the isopropylbenzene solution. Concerted homolytic C-S and S-Cl bond scission of chlorosulfonated polyethylene thermal degradation mechanism proved by kinetic analysis. The proof of the two bonds simultaneous breaking is provided by the threefold activation energy reduction (83 kJ/mol) in comparison to the C-S and C-Cl bond dissociation energy (280 and 286 kJ/mol respectively), the 6 orders lower preexponential factor (2,46 ∙ 10 s) in Arrhenius equation in comparison to one bond cleavage (≈10-10 s) as well as the strongly negative activation entropy value (-134 J/mol∙K).

Reactions of a Bis(pentalene)dititanium complex with alkenes; the molecular structure of the butadiene complex [Ti2(µ: η5,η5-Pn††)2(μ: η2,η2-s-trans-C4H6)] (Pn†† = 1,4-(SiiPr3)2-C8H4)

Reaction of the syn-bimetallic complex [Ti2(µ:η5,η5-Pn††)2] (1) (Pn†† = 1,4-(SiiPr3)2-C8H4) with 1,3-trans-butadiene in toluene results in the clean formation of the 1:1 adduct [Ti2(µ:η5,η5-Pn††)2(μ: η2,η2-s-trans-C4H6)] (2) featuring an essentially planar butadiene ligand. Complex (2) represents the first example of a bimetallic early transition metal complex where a coordinated butadiene adopts such a conformation. When (1) is reacted with propene an unexpected “tuck-over” alkene π-complex (3) is formed with co-current loss of propane. Complex (3) features a coordinated η2,η1 vinylic (H2C = CMe)-SiiPr2-Pn† moiety as a result of CH activation of one of the isopropyl substituents of the SiiPr3 groups on the Pn†† supporting ligand. One of the hydrogens of this secondary vinylic moiety is significantly shifted upfield in the 1H NMR spectrum of (3) and a single crystal XRD study shows an interaction between this hydrogen and one of the Ti centres. Preliminary kinetic studies of the formation of (3) show a slightly negative entropy of activation and 1st order consumption of (1) both of which suggest the involvement of a Ti-H(iPr) agostic interaction during the cyclometallation reaction.

Slow 3MLCT Formation Prior to Isomerization in Ruthenium Carbene Sulfoxide Complexes.

A series of photochromic complexes with general formulas of [Ru(bpy)2(NHC-SR)]2+ and [Ru(bpy)2(NHC-S(O)R)]2+ were prepared and investigated by X-ray crystallography, electrochemistry, and ultrafast transient absorption spectroscopy {where bpy is 2,2'-bipyridine and NHC-SR and NHC-S(O)R are chelating thioether (-SR) and chelating sulfoxide [-S(O)R] N-heterocyclic carbene (NHC) ligands}. The only differences between these complexes are the nature of the R group on the sulfur (Me vs Ph), the identity of the carbene (imidazole vs benzimidazole), and the number of linker atoms in the chelate (CH2 vs C2H4). A total of 13 structures are presented {four [Ru(bpy)2(NHC-SR)]2+ complexes, four [Ru(bpy)2(NHC-S(O)R)]2+ complexes, and five uncomplexed ligands}, and these reveal the expected coordination geometry as predicted from other spectroscopy data. The data do not provide insight into the photochemical reactivity of these compounds. These carbene ligands do impart stability with respect to ground state and excited state ligand substitution reactions. Bulk photolysis reveals that these complexes undergo efficient S → O isomerization, with quantum yields ranging from 0.24 to 0.87. The excited state reaction occurs with a time constant ranging from 570 ps to 1.9 ns. Electrochemical studies reveal an electron transfer-triggered isomerization, and voltammograms are consistent with an ECEC (electrochemical-chemical electrochemical-chemical) reaction mechanism. The carbene facilitates an unusually slow S → O isomerization and an unusally fast O → S isomerization. Temperature studies reveal a small and negative entropy of activation for the O → S isomerization, suggesting an associative transition state in which the sulfoxide simply slides along the S-O bond during isomerization. Ultrafast studies provide evidence of an active role of the carbene in the excited state dynamics of these complexes.

Perchlorate salts confer psychrophilic characteristics in \u03b1-chymotrypsin

Studies of salt effects on enzyme activity have typically been conducted at standard temperatures and pressures, thus missing effects which only become apparent under non-standard conditions. Here we show that perchlorate salts, which are found pervasively on Mars, increase the activity of α-chymotrypsin at low temperatures. The low temperature activation is facilitated by a reduced enthalpy of activation owing to the destabilising effects of perchlorate salts. By destabilising α-chymotrypsin, the perchlorate salts also cause an increasingly negative entropy of activation, which drives the reduction of enzyme activity at higher temperatures. We have also shown that α-chymotrypsin activity appears to exhibit an altered pressure response at low temperatures while also maintaining stability at high pressures and sub-zero temperatures. As the effects of perchlorate salts on the thermodynamics of α-chymotrypsin’s activity closely resemble those of psychrophilic adaptations, it suggests that the presence of chaotropic molecules may be beneficial to life operating in low temperature environments.

Unimolecular, Bimolecular, and Intramolecular Hydrolysis Mechanisms of 4-Nitrophenyl β-d-Glucopyranoside.

1,2-trans-Glycosides hydrolyze through different mechanisms at different pH values, but systematic studies are lacking. Here, we report the pH-rate constant profile for the hydrolysis of 4-nitrophenyl β-D-glucoside. An inverse kinetic isotope effect of k(H3O+)/k(D3O+) = 0.65 in the acidic region indicates that the mechanism requires the formation of the conjugate acid of the substrate for the reaction to proceed, with the heterolytic cleavage of the glycosidic C-O bond. Reactions in the pH-independent region exhibit general catalysis with a single proton in flight, a normal solvent isotope effect of kH/kD = 1.5, and when extrapolated to zero buffer concentration show a small solvent isotope effect of k(H2O)/k(D2O) = 1.1, consistent with water attack through a dissociative mechanism. In the basic region, solvolysis in 18O-labeled water and H2O/MeOH mixtures allowed the detection of bimolecular hydrolysis and neighboring group participation, with a minor contribution of nucleophilic aromatic substitution. Under mildly basic conditions, a bimolecular concerted mechanism is implicated through an inverse solvent isotope effect of k(HO-)/k(DO-) = 0.5 and a strongly negative entropy of activation (ΔS‡ = -13.6 cal mol-1 K-1). Finally, at high pH, an inverse solvent isotope effect of k(HO-)/k(DO-) = 0.5 indicates that the formation of 1,2-anhydrosugar is the rate-determining step.

The electronic effect of quinoline moieties on the lability of platinum(II) complexes of tridentate N^N^N and N^C^N ligands: a kinetic, mechanistic and theoretical analysis

The rate of the chloride ligand displacement by three thiourea neutral nucleophiles (Nu) of different steric demands, namely thiourea (Tu), N,N’-dimethylthiourea (Dmtu) and N,N,N,’N-tetramethylthiourea (Tmtu) in the complex 2,6-bis(8-quinolyl)-pyridine chloroplatinum(II) (Pt3), was investigated under pseudo-first-order conditions as a function of concentration and temperature using UV–visible spectrophotometry and compared with the literature data of complexes: 2,6-bis(2-pyridyl)pyridine chloroplatinum(II) (Pt1), 1,3-bis(pyridyl)phenyl chloroplatinum(II) (Pt2) and 1,3-bis(8-quinolyl)phenyl chloroplatinum(II) (Pt4). The observed pseudo-first-order rate constants for substitution reactions obeyed the simple rate law $$k_{{{\text{obs}}}} = k_{2} \left[ {Nu} \right]$$ . The results demonstrated that the lability of the chloride ligand is dependent on the degree of synergy between electronic character and the planarity of architectural frame work of the ligands around the platinum centre. The second-order kinetics and large negative activation entropies (ΔS#) assert an associative mode of activation. DFT calculations were performed to support the interpretation and discussion of the experimental data. The retardation in lability of quinoline systems; 2,6-bis(8-quinolyl)pyridine chloroplatinum(II) (Pt3) and 1,3-bis(8-quinolyl)phenyl chloroplatinum(II) (Pt4) is attributable to cis σ-donor effect and twisting of the quinoline rings that offsets the π-acceptor ability on the extended π-system. Conversely, high reactivity of pyridine systems; 2,6-bis(2-pyridyl)pyridine chloroplatinum(II) (Pt1) and 1,3-bis(pyridyl)phenyl chloroplatinum(II) (Pt2), is due to their rigid planar structure that facilitates effective π-acceptor ability.